This invention relates to drawout type protective solid state relays.
Often, switchgear equipment is provided with solid state protective relays which are used to monitor currents and/or voltages at strategic points in a power system. Such relays receive secondary signals from current and/or voltage transformers mounted in the power switchgear equipment and function to detect abnormal conditions caused by faults in the power system, such faults including insulation failures. Upon the occurrence of a fault, the protective relay generates a control signal that is used to operate the circuit breaker trip mechanism to open the breaker and deenergize the faulted portion of the power system.
A protective relay of this type is normally tested in order to ensure that it will be available when called on to perform. For the convenience of routine testing, maintenance and replacement of an in service unit, it is the practice to house the protective relay in a drawout case. This feature permits on-site testing or replacement with minimum disruption of power in the power circuit. The case itself serves as a receptacle with terminals for external connection to the relay. The case and relay have mating electrical connections, such that electrical connections are made to the relay when the relay is inserted in the case.
Desirable attributes of a true drawout protective relay wherein the relay case or housing is fixedly mounted in the switchgear are:
1. The ability to reliably and conveniently test all functions of the protective relay, independent of its housing and point of installation. PA1 2. A minimum number of parts remain in the housing when the functional unit is withdrawn therefrom. PA1 3. When the functional unit is withdrawn from its housing, the external circuits are rendered safe.
For reasons of safety, current transformer circuits must be shorted because a current transformer carrying primary current may develop an extremely high secondary voltage when open circuited. Thus, a low impedance load or a short circuit must be connected to the current transformer secondary at all times. It is necessary to disconnect operating circuits before control power is removed from the protective relay in order to prevent false operation of the switchgear equipment. Suffic1ent dielectric levels must be maintained between voltage circuits that are opened.
While the prior art recognized the necessity of incorporating the foregoing features in protective relays, achievement of these features was accomplished in a cumbersome manner. That is, in one prior art construction, before the protective relay could be withdrawn from its housing, it was necessary to operate a plurality of switches located on the front panel of the relay in order to short current transformer secondaries. In another prior art construction, the drawout section could not be withdrawn until a so-called shorting bar was removed from the front panel. Removal of such shorting bar permitted current transformer shorting contacts to close.